https://github.com/matplotlib/matplotlib/issues/1781

Using multiprocessing on Travis was causing "Permission denied" errors -- but maybe it no longer does that, since clearly we're able to run our tests in parallel now.

Note, however, that this only applies to 1.2.x -- 1.3.x doesn't parallelize 2to3 because distribute is doing the 2to3 for us over there, and it doesn't support it -- it's most likely possible by overriding something or monkey-patching, but I haven't really spent any time looking into it.